Here's some information that may help if you find your cat has an adversion to a litter box. I found it on: http://www.hsus.org/pets/pet_care/our_pets_for_life_program/cat_behavior_tip_sheets/solving_litter_box_problems.html
"Keep the litter box extremely clean. Scoop at least once a day and change the litter completely every four to five days. If you use scoopable litter, you may not need to change the litter as frequently, depending on the number of cats in the household, the size of the cats, and the number of litter boxes. If you can smell the box, then you can be pretty sure it's offensive to your cat as well.
Add a new box in a different location, and use a different type of litter in the new box. Because your cat has decided that her old litter box is unpleasant, you'll want to make the new one different enough that she doesn't simply apply the old, negative associations to the new box.
Make sure that the litter box isn't near an appliance (such as a furnace) that makes noise, or in an area of the home that your cat doesn't frequent.
If ambushing is a problem, create more than one exit from the litter box, so that if the "ambusher" is waiting by one area, your cat always has an escape route.
If you have multiple cats, provide one litter box for each cat, plus one extra box in a different location."